1. Field of the Invention
The present invention relates generally to surface treatments to facilitate the processing of lithium (or other alkali) metal or alloys, such as for incorporation in electrochemical devices.
2. Description of Related Art
Lithium is an attractive material for use as an electrode component in electrochemical devices, such as batteries and capacitors, due to its very high energy density and low equivalent weight. However, lithium is highly reactive in ambient conditions and thus requires special handling during processing. Typically, lithium battery manufacture is conducted in inert environments in order to guard against degradation of lithium until it is hermetically sealed within a battery cell container.
Even with these precautions, lithium may detrimentally react with incompatible materials in the processing environment. For example, rechargeable lithium metal batteries have been prone to cell cycling problems. On repeated charge and discharge cycles, lithium “dendrites” have been found to gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow “mossy” deposits which can dislodge from the negative electrode and thereby reduce the battery's capacity. To address these problems, some researchers have proposed that the electrolyte facing side of the lithium negative electrode be coated with a “protective layer.” Several methods may be envisioned for producing such a protective layer, but the processing methods by which such layers are produced may not be compatible with the lithium metal.
Some research has focused on “nitridation” of the lithium metal surface as a means for protecting lithium electrodes. In such process, a bare lithium metal electrode surface is reacted with a nitrogen plasma to form a surface layer of polycrystalline lithium nitride (Li3N). This nitride layer conducts lithium ions and at least partially protects the bulk lithium of the negative electrode from a liquid electrolyte. A process for nitriding lithium battery electrodes it is described in R&D Magazine, September 1997, p. 65 (describing the work of S. A. Anders, M. Dickinson, and M. Rubin at Lawrence Berkeley National Laboratory). Unfortunately, in addition to structural and electrical problems with this approach, lithium nitride decomposes when exposed to moisture. While lithium metal batteries employ nonaqueous electrolytes, it is very difficult to remove all traces of moisture from the electrolyte. Thus, trace moisture will ultimately compromise the protective properties of the lithium nitride.
Other pre-formed lithium protective layers have been contemplated. Most notably, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (“LiPON”) or related material. LiPON is a single ion (lithium ion) conducting glass. It is typically deposited by reactive sputtering of a lithium phosphate in the presence of nitrogen. The nitrogen, however, attacks the lithium surface, thereby making the process of direct deposition of the glass film impossible.
Other examples of potential protective layers may include the deposition of polymer layers that involve solvents or monomers that are incompatible with lithium.
Accordingly, it would facilitate handling of metallic lithium, lithium alloy or other alkali metal or metal alloys to provide an adequate surface protective layer. In particular fabrication processing and successful operation of alkali metals as battery electrodes would be enhanced by the provision of such a protective layer.